Aircraft are used in a wide variety of applications, both civilian and military, including travel, transportation, fire fighting, surveillance, and combat. Various aircraft have been designed to fill the wide array of functional roles defined by these applications, including balloons, dirigibles, traditional fixed wing aircraft, flying wings and helicopters.
In general, aircraft travel at a sufficient altitude to substantially eliminate any threat posed to the aircraft from threats posed by personnel on the ground. For some applications, however, it is necessary to travel at comparably low altitudes for long periods of time, exposing the aircraft to threats from the ground. Similar exposure takes place in other applications during take-offs and landings of the aircraft.